Women who feel stressed at work are at a higher risk of developing breast cancer, according to a new study.

Researchers found that women in more demanding jobs are 30 percent more likely to develop breast cancer than those who feel they are on top of their work.

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The 14-year study involved 36,000 Swedish women, aged 30 to 50, who were working since the study started in 1990. The study concluded in 2004 and found that 767 of the volunteers had been diagnosed with breast cancer.

The number of breast cancer cases increased by approximately 30 percent for women with stressful jobs. Other factors such as alcohol consumption, number of children, weight and age, were also taken into account. Researchers found no link between stress and cancer among women in part-time work.

The reason why stress might increase the risk is unclear, although studies show it may raise levels of the estrogen which can increase the risk of cancer.